The present invention relates in general to parachute design and manufacture, and in particular to a new and useful method of making a parachute which utilizes a particular configuration of reinforcement lines and wherein even nonporous material can be used for the parachute canopy.
Conventional round textile parachutes are usually made in the following manner: First, gores each having the form of a circle segment, are made either by direct cutting out of cloth or sewing from smaller pieces. From these gores the canopy is sewn together either to form a flat circle or a shallow cone, depending on the number of gores and the segment angle of each gore. The possible reinforcements are added at this time into the canopy and some more reinforcements are added when the suspension lines are sewn into the canopy. The suspension lines either can continue from the skirt over the canopy vent back to the opposing skirt or they are attached only to the skirt. In the latter case the gore seam can be reinforced some more. The reinforcements are naturally put on such places, where the strength of the original structure is insufficient compared to the stresses which will be placed on the parachute in use.
The manufacturing technique need is also dictated by the width and price of the cloth used. The canopy must be put together from smaller parts and with little leftover. From this point of view for example plastic film would be a more suitable material. It is available in larger widths (several yards) and for a fraction of the cost of textile cloth. However, plastic films have properties, which make it useless as parachute material. It is fully nonporous, wherefrom follows an increased opening shock and the already low strength of the film cannot stand the increased stresses. A nonporous canopy is also unstable, which makes it unsuitable for some purposes. Plastic film cannot be sewn, but one must use manufacturing techniques suitable for the material, like heat seaming or glueing.
U.S. Pat. No. 4,326,683 to the present inventor, as well as U.K. patent 1,587,309 and Finnish patent 54,893, disclose an invention where a canopy of nonporous material can be changed with addition of a particular porosity, into a stable parachute with so smooth an opening, that plastic film can be used as the parachute material in some cases. The problems in manufacturing, like reinforcing the canopy, however, are not solved by this invention.
The stresses in the canopy follow from aerodynamic pressure loading, which is the pressure differential from the inside to the outside of the opening canopy. The first structural element taking this loading is the canopy material, which stresses both in the direction of the suspension lines (radially) and perpendicular to them (tangentially). The radial stresses are transferred into the suspension lines and the tangential stresses open up the canopy and load mainly the canopy material. The suspension lines transfer the loading to the suspended load in the confluence point. The distribution of the pressure loading in the canopy area is a very complicated phenomenon, which is theoretically not well known. During the opening phase, when the loading is highest, the air flow is dynamic in the surrounding and through an elastic and porous body, which flow is partly laminar and mainly turbulent and separated from the body.